MG will be running the MG6 again but new colour scheme as Plate has moved to BMR.
They wanted to use the tourer for two reason. Honda wanted to advertise the new car, and they though it would provide more straightline speed. They had the IVTEC engine for the first time last year so that was being developed. Matt Neal had a shocker of a season so I expect he'll be on the pace this year providing they don't have any new new car teething issues.
The Merc was Adam Morgan whose family run Ciceley Racing team who has sponsors from WIX hence the yellow. They're the same this year but Aidan Moffat has made the swap from a chevvy cruze to the Merc that's been prepped by Ciceley for him and they're working together. He's got the best looking livery with his laser tools sponsorship I've seen so far this year but most of them will release them on media day.
The audi was Rob Austin Racing who is now more orange and black (hes used to have the WIX sponsorship but now has Exocet now as his main) runs a pair of A4's. They're a fan favourite but i think the new rules to slow down the rear wheel drive cars may hurt him the most.
Other big chances are the BMW's have lost the eBay sponsorship and its not been confirmed but I don't think tesco's are doing any backing this year. Last year the MG manufactures team had a mouthful of a title "MG KX Clubcard Fuel Save".
We've got a new manufacture backing this year in the form of Infiniti who are fun by Support Our Para's (Red Bull did some good stuff today for that team
http://www.btcc.net/2015/03/18/infiniti-teams-join-forces-for-the-day/ ).
Other big moves are Motorbase who run the ford focus will not be out till half way into the season due to not finding a title sponsor. United Autosports will not be running, PowerMaxx just sponsored a team last year, this year they've purchased that team and are running two chevvy cruzes. Going to be another good looking grid thats for sure!